1. Field of the Invention
The present invention relates to an acceleration sensor which utilizes a semiconductor technique.
2. Description of the Related Art
Recent vehicles are provided therein with an air bag system for the safety of the drivers, assistants and the like. The air bag system is a system for inflating the air bag placed in front of or beside the driver or the like when the acceleration becomes too large due to a collision or the like. In this system, therefore, a means for detecting the acceleration is essential for the purpose of triggering the action. As such an acceleration detecting means, it is possible to use an acceleration sensor which is composed of a semiconductor substrate as well as a conventional ball contact type acceleration sensor. Between the two, the former is expected to find a wide use not only in an air bag system but also as a means for detecting an acceleration because it is produced by applying a semiconductor technique and it is integrated into various semiconductor circuits.
FIGS. 21A and 21B show the structure of a conventional acceleration sensor. This acceleration sensor is disclosed in Nikkei Electronics, 1991, Nov. 11, No. 540, p. 224, etc.
Referring first to FIG. 21A, which shows the structure of the acceleration sensor and the circuit attached thereto. In the conventional acceleration sensor, a cantilever spring structure of polysilicon is formed on a silicon substrate 10.
The cantilever spring is provided with a pair of columns (leaf springs) 14 each having an anchor 12 at both ends thereof. The anchors 12 are fixed to the Si substrate 10 by a fixing member 16 made of polysilicon which is provided with conductivity. The columns 14 are therefore fixed above the Si substrate 10 with a predetermined space therebetween.
On the other hand, the columns 14 support a weight 18 made of polysilicon. The pair of columns 14 are placed in parallel on the Si substrate 10 in the state of being connected by the weight 18, as shown in FIG. 21A. When an acceleration is applied to the weight 18 in the direction indicated by the arrow in FIG. 21A, the weight 18 is deflected in the same direction as the direction of acceleration due to the elasticity of the columns 14. As will be described later, as the acceleration is indirectly detected from this deflection, the axis in the direction of the arrow is called a detection axis.
The weight 18 is provided with movable electrode plates 20 which are made of polysilicon in the shape of ribs. Fixed electrode plates 22 and 24 are fixed to the Si substrate 10 in the same way as shown in FIG. 21B. The fixed electrode plates 22 and 24 are arranged in pairs such that each pair of them meshes with the corresponding pair of rib pieces of the movable electrode plates, in other words, such that the distance between each pair of the fixed electrode plates 22, 24 and each pair of the rib pieces of the movable electrode plates 20 changes when the movable electrode plates 20 are deflected with the deflection of the weight 18. Therefore, when an acceleration is applied to the weight 18 in the direction of the detection axis, the distance between the fixed electrode plate 22 and the rib piece of the movable electrode plate 20 changes, resulting in a change of the capacitance.
As a circuit attached to such a structure, a circuit including a rectangular wave generator 26 of 1 MHz, a buffer amplifier 28, a modulation and low-pass filtering unit 30, a reference voltage generator 32, and a preamplifier 34 are shown in FIG. 21A. When 1 MHz voltages of opposite phases VA and VC each having a rectangular shape are applied from the rectangular wave generator 26 to the fixed electrode plates 22 and 24, a voltage VB is obtained from the columns 14 which are in contact with the movable electrode plates 20. Although the wirings between the movable electrode plates 20 and the fixed electrode plates 22, 24 in front of the weight 18 are shown in FIG. 21A, the similar wirings are actually conducted on those elements at the back of the weight 18.
Since the voltage VB is caused by the deflection of the movable electrode plates 20 which constitute a differential capacitor, the value indicates the deflection of the weight 18 due to the acceleration, as shown in FIG. 22A. The voltage VB is input to the modulation and low-pass filtering unit 30 through the buffer amplifier 28, and the output of the modulation and low-pass filtering unit 30 is input to the circuit at the next stage by the preamplifier 34 to which an offset is given by the reference voltage generator 32.
In this way, the conventional acceleration sensor which can be produced by using a semiconductor technique is realized.
The acceleration sensor having the above-described structure, however, has the following problems. The complicated structure makes the production difficult, which leads to a high price. Since the capacitance related to the detection of the acceleration is minute, it is impossible to secure the stability of the detection characteristic when the acceleration changes, and the sensitivity of the sensor is low.